Merge pull request #15454 from webpack/fix/issue-15447

use cache in BannerPlugin

lib/BannerPlugin.js

@@ -77,6 +77,7 @@ class BannerPlugin {
 			undefined,
 			options
 		);
+		const cache = new WeakMap();
 
 		compiler.hooks.compilation.tap("BannerPlugin", compilation => {
 			compilation.hooks.processAssets.tap(
@@ -102,10 +103,15 @@ class BannerPlugin {
 
 							const comment = compilation.getPath(banner, data);
 
-							compilation.updateAsset(
-								file,
-								old => new ConcatSource(comment, "\n", old)
-							);
+							compilation.updateAsset(file, old => {
+								let cached = cache.get(old);
+								if (!cached || cached.comment !== comment) {
+									const source = new ConcatSource(comment, "\n", old);
+									cache.set(old, { source, comment });
+									return source;
+								}
+								return cached.source;
+							});
 						}
 					}
 				}

test/BannerPlugin.test.js

@@ -0,0 +1,41 @@
+"use strict";
+
+const path = require("path");
+const fs = require("graceful-fs");
+
+const webpack = require("..");
+
+it("should cache assets", done => {
+	const entry1File = path.join(__dirname, "js", "BannerPlugin", "entry1.js");
+	const entry2File = path.join(__dirname, "js", "BannerPlugin", "entry2.js");
+	try {
+		fs.mkdirSync(path.join(__dirname, "js", "BannerPlugin"), {
+			recursive: true
+		});
+	} catch (e) {
+		// empty
+	}
+	const compiler = webpack({
+		mode: "development",
+		entry: {
+			entry1: entry1File,
+			entry2: entry2File
+		},
+		output: {
+			path: path.join(__dirname, "js", "BannerPlugin", "output")
+		},
+		plugins: [new webpack.BannerPlugin("banner is a string")]
+	});
+	fs.writeFileSync(entry1File, "1", "utf-8");
+	fs.writeFileSync(entry2File, "1", "utf-8");
+	compiler.run(err => {
+		if (err) return done(err);
+		fs.writeFileSync(entry2File, "2", "utf-8");
+		compiler.run((err, stats) => {
+			const { assets } = stats.toJson();
+			expect(assets.find(as => as.name === "entry1.js").emitted).toBe(false);
+			expect(assets.find(as => as.name === "entry2.js").emitted).toBe(true);
+			done(err);
+		});
+	});
+});